1. Field of the Invention
The present invention pertains to a digital signal transmitting circuit for a telephone system known as an integrated services digital network.
2. Description of the Prior Art
This network has telephone lines between telephone exchanges and subscriber stations in order to transmit telephone conversations among subscribers as well as other data flowing through in the form of digital signals.
According to the standard CCITT 1430, each subscriber is connected to the network by an interface element called a "network termination" which is coupled firstly to the two wires of a telephone line of the network and, secondly, to a local four-wire network (a two-wire transmission bus and a two-wire receiver bus). The local network connects the interface element to local terminals at the disposal of the subscriber. These local terminals may include a telephone, a console for alphanumeric link-up in conversational mode, etc.
The standard referred to above defines the various functions and characteristics required of the interface element.
In particular, this element has a so-called "U interface circuit" which is connected to the two wires of the telephone line by a transformer, a so-called "S interface circuit" which is connected to four wires of the local network by means of a transmitting transformer and a receiving transformer and, finally, between these two interface circuits, a signal processing circuit which makes the signals that are transmitted or received on two wires compatible with the signals transmitted and received on four wires.
As regards the S interface circuit, the standard defines, in particular, the limits set on the digital signals transmitted to the local terminals in different types of transmission as well as the impedances of the circuit as seen from the transmitting bus, the said impedance taking into account the transmitting transformer placed between the S interface circuit and the transmitting bus.
More specifically, the impedance should be at least 2.5K ohms between 20 and 80 KHz for a transmitted voltage level of zero (this level corresponds to a binary logic level 1 in the type of digital coding used, according to which the binary state 1 is defined by a voltage level of zero and the binary state "0" is defined alternately by a voltage level of +0.75 volts and then by a level of -0.75 volts).
To achieve the final driving stage of the transmitting transformer, several methods may be considered. These methods prove to have major disadvantages.
These methods are shown in FIGS. 1a to 1c.
The method of FIG. 1a lies in the use of a transformer, the primary coil of which has a midpoint, carried to a reference potential V.sub.O, and two ends, each of which is connected to a switch (I1, I2 respectively) which may either stay open or connect the corresponding end of the primary winding to a potential which is V1=V.sub.0 +1.5 volts (it being assumed that the transformer lowers the voltage levels applied to the primary coil by half in order to obtain pulses of +0.75 volts or +0.75 volts at the secondary coil).
The potential V.sub.O is applied by a buffer amplifier with a unit gain A.sub.O. The potential V.sub.O +1.5 volts is applied by a buffer amplifier with a unit gain Al.
This method can be used in MOS technology but requires a transformer with a midpoint. It raises difficulties in controlling the edges of the signals to be transmitted and, finally, this method does not make it possible to maintain all the pulse shape limits stipulated by the CCITT standard I430. For the edges of the signals are created by the closing of the switches I1 and I2 and are far too steep, so that they create overvoltages which do not comply with the set limits.
FIG. 1b is a diagram with a transformer without midpoint, which can be used in the MOS technology, but also has the same disadvantages as the method of FIG. 1a. Furthermore, this arrangement might create a functional dissymmetry if the voltage sources used are not exactly equal to V.sub.O +1.5 volts and V.sub.O -1.5 volts.
The arrangement shown in FIG. 1c makes it possible to improve the quality of the edges of the signals transmitted. The outputs of the amplifiers are applied directly to the ends of the windings (without using switches) to transmit appropriate levels of voltage to the primary winding of the transformer.
The amplifier receives feedback so that it can hold the set limits of the pulse shapes if the impedance, seen from the transmission point, is 50 ohms or 400 ohms (the standard CCIT 1430 stipulates a set limit for 50 ohms, 400 ohms and 5.6 ohms).
However, the dynamic range of the output of the amplifier should be such that its output can go from 1 volt to 4 volts (1.5 volts on either side of a midpoint at 2.5 volts). These values are too close to the power voltage (0-5 volts) of the amplifier and, since a large amount of current of about 7.5 mA has to be given on 200 ohms, this would require very big transistors in the output stage of each of the two amplifiers used. As in the diagram of FIG. 1p there is a problem of dissymmetry if the reference voltage sources have wrongly adjusted levels.
Other methods have been suggested, but they use bipolar technology and cannot be transposed to MOS technology as they use properties specific to bipolar elements which cannot be transposed to MOS elements, such as, for example, current limitations imposed by diodes which shunt a part of the bipolar transistors' base current.
Now, in certain cases, it is sought to make this transmission circuit using MOS technology, if only because the rest of the interface element has a great deal of logic elements which it would be preferable to make with low-consumption technology (MOS or better still CMOS).